Geometric Upper Critical Dimensions of the Ising Model

The upper critical dimension of the Ising model is known to be d_c = 4, above which critical behavior is regarded to be trivial. We hereby argue from extensive simulations that, in the random-cluster representation, the Ising model simultaneously exhibits two upper critical dimensions at (d_c = 4, d_p = 6), and critical clusters for d ≥ d_p, except the largest one, are governed by exponents from percolation universality. We predict a rich variety of geometric properties and then provide strong evidence in dimensions from 4 to 7 and on complete graphs. Our findings significantly advance the understanding of the Ising model, which is a fundamental system in many branches of physics.